How much do I gain from switching to it instead of say, remaining on the Stock Android?
Edit: This looks comprehensive — https://staging.grapheneos.org/features
This is an OS for people who care more about privacy and security than having an everyday usable phone. It is very much not for normal people.
Compatibility with carriers also improved a lot a few years ago. Configurations for most carriers are pulled in from the stock Pixel OS. Some US carriers do weird things that depend upon having highly privileged apps bundled into the OS which, for security reasons, GrapheneOS doesnt include. I dont recall AT&T being one of them.
GrapheneOS is very usable and fine as a everyday phone for normal people.